This invention relates generally to electric cooking appliances and particularly to electric cooking appliances incorporating electronic power control systems.
Commonly assigned U.S. Pat. No. 4,282,422 to Payne et al discloses a power control system for electric cooking appliances using infrared heating units made of resistive materials characterized by high positive temperature coefficient of resistivity, low thermal mass, and low specific heat, such as molybdenum disilicide and tungsten. The power control system therein described addresses the control problem presented by the dynamic thermal and electrical characteristics of such heating units in which the resistance typically increases by an order of magnitude as the temperature of the unit increases from room temperature to operating temperature and which changes temperature rapidly (the first thermal time constant is on the order of 600-1000 milliseconds). The power control system provides rapid switching and uses repetition rate control to space the brief ON times associated with the lower power settings so as to limit cooling between ON times to prevent excessive current surges.
Power control for each heating unit is accomplished by generating a multi-bit digital control word for each unit representing the power level to be applied to the unit. The state of each bit determines whether power is to be applied to the associated unit for one control interval. The word for each heating unit is updated once each control period. Each control period comprises a number of control intervals equal to the number of heating units. Power control for each unit is staggered by testing a bit of a different positional value in each word during any one control interval. The resultant staggering of energization of the heating units tends to reduce the total current drawn by the multi-unit cooking appliance during any one control interval.
Infrared heating units designed for energization by a 120 volt AC supply rather than a 240 volt supply can be fabricated using less costly, more durable resistance wire resulting in less expensive and more reliable units. However, achieving the same output power at the lower voltage obviously requires substantially higher operating currents. Due to the aforementioned dynamic characteristics of the resistance wire used in such units, the instantaneous peak currents can be undesirably high, particularly in the common neutral line when operating multiple heating units especially at the lower power settings.
The '422 staggering approach effectively limits total peak load current drawn by the appliance at the lower power settings, particularly for the current levels typical of 240 volt operation. However, the high peak neutral line current associated with 120 volt operation under certain operating conditions may not be adequately limited by the staggering approach. Hence, there is a need for a control arrangement which satisfactorily limits peak neutral line current over a relatively wide range of power settings to permit fuller realization of the cost savings achievable through use of heating units designed for energization at 120 volts.
It is therefore an object of the present invention to provide an electric cooking appliance of the type having multiple heating units connected to a common neutral power line in which the application of current pulses to each heating unit is controlled so as to minimize the instantaneous peak current in the neutral power line.